Hal Clement's novel, Ice World, is about sulfur breathing aliens who visit our planet, which has lots of ice, hence the title. I wanted to ask about a nifty device he has. I forgot to ask him about it when I had the chance.

The aliens have something called a swap box. It's a clear box with two compartments with lids; each lid slides back and forth. You put something in one and close it, and closing it opens the other compartment. You can't open the closed compartment, until you've put something in the open compartment.

Has anybody actually made one of these?

I found a quote from the novel itself:

They find "swap box" (by opening a compartment near the bottom of torpedo) - "two trays, hinged together, each divided into a number of small compartments. One side is empty, while the compartments of the other are filled with various articles that are for sale. A glass lid covers each of the full compartments, and cannot be removed until something has been placed in the corresponding compartment of the other tray." A few iterations, & both parties know what the other values - tobacco for platinum! In the process, aliens also learn the sound of a very few English words, & guess their meaning.